DOCSP-7346: [Guides] Update Atlas Guide for 'three door' UI (#135)

Author: steveren

source/images/firstcluster.png

@@ -1,4 +1,4 @@
-title: Create or login to an Atlas user account
+title: Create or log in to an Atlas user account
 ref: process-mongodb
 level: 4
 stepnum: 1
@@ -8,7 +8,7 @@ content: |
   
   Go to `cloud.mongodb.com
   <https://cloud.mongodb.com?utm_source=atlas-account-guide&utm_campaign=guides&utm_medium=docs>`_
-  to create or login to your user account.
+  to create or log in to your user account.
 
   If you are creating your user account, as part of the process Atlas
   automatically creates a default "organization" and "project" for you.
@@ -50,9 +50,20 @@ content: |
   Now it's time to select your cluster tier. In this guide we will be
   setting up a free-tier or ``M0`` cluster.
 
-  Ensure that the ``Cloud Provider and Region`` dropdowns have ``Amazon
-  Web Services`` and either ``N. Virginia (us-east-1)`` or ``Frankfurt
-  (eu-central-1)`` selected.
+  Select the left-side panel to proceed with your free-tier cluster
+  setup.
+
+  .. figure:: /images/starter-clusters.png
+     :figwidth: 200px
+
+  .. note::
+
+     If you already have one or more clusters in your Atlas project,
+     you'll go straight to the next screen. 
+
+  On the next screen, ensure that the ``Cloud Provider and Region``
+  dropdowns have ``Amazon Web Services`` and either ``N. Virginia
+  (us-east-1)`` or ``Frankfurt (eu-central-1)`` selected.
 
   .. figure:: /images/cloudselect.png
      :figwidth: 700px